In large print systems multiple calibrations are performed by sensing the position of printed marks and making adjustments based on the results of these measurements. Often multiple sensors are employed to perform each of the calibrations because the required qualities of the sensors, for example, resolution, differ from application to application.
In a print system with wide receivers it is often necessary to align multiple print elements so they can function as one wide element to span the width of the receiver. For example, in large inkjet printers, multiple 6″ wide lineheads are combined to print on 19″ or 25″ wide paper. Since the lineheads cannot be mounted end to end they are offset from each other in the direction of media travel. To print a straight line of data on the paper, the printing on each linehead must be enabled at different times so that the image is printed in alignment on the receiver. This timing adjustment produces alignment in the direction of media travel.
Due to mechanical tolerances, there must be a certain amount of overlap between lineheads in the cross travel direction. Alignment in the cross travel direction is achieved by selecting the printing elements on which one linehead stops printing and the next linehead starts printing. A method to align the lineheads is to print marks from each linehead, measure the marks, and adjust the exposure timing and overlap pixel for optimal printing. A common method to do this is to use high resolution digital cameras to measure marks from each linehead and make the adjustments.
For a high quality print all the color planes should be printed directly on top of each other. Any error is called misregistration and is unacceptable. To maintain good registration the positions of the colors are measured regularly and adjusted. A final group of functions include the detection of defects such as streaks or missing lines of data and the visualization of images as they are printed.
Current implementations use multiple sensors for these functions. For example, multiple high resolution cameras with small fields of view can be used for the first two functions while a line array with a wide field of view can be used for the third function. It is not practical to acquire the full width at high resolution because it becomes very expensive to handle the large amount of high speed data.